I do not think that the mystery items(XP & Gold) are a good direction for TFT.
The XP can really ruin your early game(possible late game build) by not getting tier 1 units like elise and graves. The pace of the game is very very fast now compared to how it is on live. XP gain from mystery items can really hurt a player. You can be getting blue and green units before you can even complete a tier 2 grey unit. That hurts a players economy and gameplay because they have to spend 2-3 gold on blue/green units now, because grey ones do not spawn as much. It also cost the player a possible T3 champ late game because they were not able to get that grey unit early game. Just because you can get an extra XP and a high level does not mean it is good for the player.
The gold isn't necessarily bad, but most players would rather have items than gold. Gold should stay as part of the game economy saving mechanic.
Keeping the old system(on LIVE), where just random items drop after PvE rounds is probably the best direction for TFT at the moment. It is already a balanced game for the most part, except for item numbers and item stacking(but that is another story), just balance item spawn numbers to be fair for everyone.